In the text where you want the footnote to appear, you put <ref> and </ref> around the material to be placed in the footnote. Then under some reasonable heading at some reasonable place in the document, you put <references/> where you want the footnotes to appear. You may use plain text, an internal link, an external link,  or a combination including any or all of the above.
As with every page on the wiki, you may see the source code by clicking on the edit tab at the top of the page. It won't hurt anything to take a look at the innards of the pages. The page is not locked while you are in edit mode. Other people can look at the page and can even click on the edit tab at the same time. When you're done, click cancel--unless, of course, you have made an edit that will improve the page.